1. Field of the Invention
The present invention concerns a method of forming coatings in coated tubular metal members having relatively small diameter of about less than 20 mm and thin wall thickness, which are generally disposed to outer portions such as floor surface as various pipelines for supplying oils and airs in automobiles and like other machineries.
2. Description of the Prior Art
Coatings in coated tubular metal members of this kind for satisfying simultaneous requirements for protection against external impact shocks given by pebbles sprung up during running and corrosion resistance together have been formed, for example, by a method of coating the outer circumferential surface of a tubular metal member applied with usual zinc plating treatment having chromate membrane directly with a tubular material such as made of a heat shrinkable vinyl chloride resin or polyolefin resin and, thereafter, applying heat treatment by passing them through a furnace to cause heat shrinking of the tubular material and thereby depositing and laminating the them.
However, in such a prior art, since the coating structure comprises a single layer of heat shrunk tubular material, the coating necessarily has a relatively large thickness of about 100 um order, to remarkably hinder the workability in the subsequent complicate bending fabrication for obtaining a final product and a further improvement is also demanded for the corrosion resistance. Further, the coating operation for the tubular material is troublesome in long tubular metal members to result in the reduction of the productivity and tending to increase the product cost. Further, the coating in the deposited and laminated state generally lacks in close bondability with the tubular metal member, thus causing gaps in the bent portion upon bending fabrication or at the coating ends due to peeling to bring about problems such as intrusion of dusts and water of rainfall and car washing.